Generally, in a color cathode ray tube, if any unwanted magnetic field from terrestrial magnetism or external circuits enters the passing zone of three electron beams of three primary colors, blue (B), green (G), and red (R), the three electron beams tend to break away from their original tracks due to the effect of the unwanted magnetic field. This is called miss-landing. The miss-landing causes the degradation of color purity in a color cathode ray tube. For this reason, color cathode ray tubes ordinarily contain an inner shield of a magnetic shielding material inside of the funnel where the three electron beams are easily affected by magnetic fields.
The inner shield has the basic structure of the hollow shape. However, such basic structure may not shield external magnetic fields sufficiently. Therefore, further developed inner shields with various shapes and structures have been introduced to shield external magnetic field more appropriately.
For example, Korea Patent Application No. 1997-029742 discloses an inner shield for color cathode ray tube. As illustrated in FIG. 1, the inner shield 10 comprises long side parts 15, short side parts 17 and corner parts 19, which form the large opening part 11 and the small opening part 13. The long side parts 15 and the short side parts 17 are inclined towards the direction of the tube axis Z. At the small opening part 13's side of the long side parts 15 and the short side parts 17, the triangular extension members 151, 171 are connected respectively as one body. Also, at the small opening part 13's side of the corner parts 19 between the long side parts 15 and the short side parts 17, the extension members 191 are connected. In the long side parts 15 and short side parts 17, the triangular notches 153 and 173 are formed respectively. The notches 193 are also formed in the corner parts 19. Here, the extension members 151 and notches 153 are for concentrating magnetic flux at the long side parts 15. Also, the extension members 171 and notches 173 are for concentrating magnetic flux at the short side parts 17. Furthermore, the extension members 191 and notches 193 are for concentrating magnetic flux at corner parts 19.
Japanese Patent Laid-Open No. Hei 5-159713 also introduces an inner shield. As illustrated in FIG. 2a, FIG. 2b, and FIG. 2c, the inner shield 20 comprises long side parts 23 and short side parts 24, which form the large opening part 21 and the small opening part 22. On the small opening part 22's side of the long side parts 23 and the short side parts 24, recessed parts 29 and 30 are formed respectively. At the four diagonal points of the small opening part 22, the recessed part 29 makes an angle θv with respect to the center axis 25 of the long side part 23. The recessed part 30 also makes an angle θv with respect to the center axis 26 of the short side part 24. The depth of the recessed parts 29 and 30 is HD-HV.
Additionally, Japanese Patent Laid-Open No. Hei 11-354040 also introduces an inner shield. As illustrated in FIG. 3, the inner shield 40 comprises long side parts 43 and short side parts 44 to form the large opening part 41 and the small opening part 42. At the four diagonal points 45 of the small opening part 42, the recessed parts 46 and 47 are formed on the long side parts 43 and the short side parts 44, respectively. At the point 45, the recessed part 46 makes an angle α with respect to the center axis 48 of the long side part 43. The recessed part 47 makes an angle β with respect to the center axis 49 of the short side part 44 at the point 45. The long side part 43 makes an angle δ from the bottom surface of the large opening part 41 and the short side part 44 makes an angle δ from bottom surface of the large opening part 41. The diagonal axis 50 makes an angle ε to the surface of the large opening part 41. The edge 51 of the diagonal axis 50 at the small opening part 42 side makes an angle ε′(≦ε) to the surface with the depth d1 or d2 at a right angle to the tube axis Z. The surface with the depth d1, d2 at a right angle to the tube axis Z forms the angle δ′(≦δ) to the long side parts 43 and the short side parts 44.
Notwithstanding the various shapes and structures of the conventional inner shields, the unwanted magnetic field from terrestrial magnetism or external circuits has not been sufficiently shielded from entering the electron beam passing zone of the color cathode ray tube and miss-landing has frequently occurred. Consequently, the color purity was degraded and it was difficult to obtain high quality images.